Large valued single input signals typically present problems for fully differential operational amplifiers. When one of the inputs of a differential amplifier is grounded and a single input signal is applied to the other input, there is theoretically no problem associated with this arrangement. However, the input common-mode voltage of the differential amplifier is therefore equal to one-half of the input signal which is referenced from ground. In actuality, due to a finite common-mode input voltage range which every realizable differential amplifier has, the presence of a large input voltage can result in a common-mode input voltage which is greater than the differential amplifier can accomodate. If the common-mode input voltage is exceeded, an undesirable delay occurs before the output voltage is resolved and provided by the differential amplifier. Therefore, the settling time of a differential amplifier may increase above a tolerable value. Also, the common-mode input voltage is susceptible to much variation in this type of circuit operation.
To avoid these problems, others have adapted a fully differential amplifier by using a switched capacitor input structure to accept a single input signal and convert the signal to a differential signal. For example, a switched capacitor fully differential amplifier is shown and described in "A Family of Differential NMOS Analog Circuits for a PCM Codec Filter Chip" by Senderowicz, Dreyer, Huggins, Rahim and Laber, in the IEEE Journal of Solid-State Circuits, Volume SC-17, No. 6, Dec. 1982, pages 1014-1021. In general, such operational amplifiers are parasitic prone in that parasitic charge is coupled to the operational amplifier and results in an error voltage. Further, the use of some switched capacitor structures may vary the transfer characteristics of the amplifier structure which may detrimentally affect voltage gain and frequency response.